this serves a useful purpose in piling up the free air in the cylinder
before the inlet valve closes on the return stroke.

[Illustration: FIG. 17.--COMBINED STEAM AND AIR INDICATOR CARD:

Taken from a 16x18 Sergeant piston inlet air compressor, meyer's cut-off
at 3/10. Steam at 58 lb.; air pressure, 77 lb.; total engine friction, 5
per cent.]

Fig. 17 illustrates a combined steam and air indicator card taken from
one of these cylinders. It will be observed that with steam and air
cylinders equal in diameter and stroke, an air pressure of 77 pounds is
reached with a steam pressure of only 58 pounds. The reason for this is
plainly shown in the cards, their areas being nearly equal. What is made
up in the air card by high pressure is represented in the steam card by
greater volume. The indicated efficiency deduced from these cards is 95
per cent., that is, the area of the air card divided by the area of the
steam card, representing the resistance divided by the power, results in
95 per cent. While several cards have been taken on the cylinders
showing a loss by friction of only 5 per cent., yet on the average the
best practice shows a loss of 6 per cent. or an efficiency of 94 per
cent. This result indicates an almost perfect proportion between power
and resistance, and good workmanship in air-compressing machinery. It is
difficult to conceive an engine of this size being worked with a less
expenditure for friction than 5 or 6 per cent. Were it possible to
retain the heat which is in the air, and which is represented by the
space between the dotted isothermal curve and the actual curve, we might
attain high efficiency in using compressed air power, but it is evident
that the power represented by the area of this space will be lost by
radiation of heat before it is used in an engine situated several